In normal breast, 10-20% of luminal epithelial cells express estrogen receptor alpha (ERa) and these cells correspond to non-dividing cells located adjacent to ERa-negative epithelial cells. Yet 55% of breast cancers express ERa and proliferation of these cancers depends on ERa as evidenced by response to hormonal therapy. One possible explanation for this conundrum is that checkpoint-mediated failsafe mechanisms such as senescence prevent ERa and its ligand, estrogen (E2), from inducing proliferation and loss of this barrier is one of the switches that triggers ERa-dependent proliferation. Unfortunately, unlike the well-documented in vitro transformation model for ERa-negative breast cancers, no in vitro model system to dissect pathways involved in transformation of ERa-positive mammary epithelial cells exists because only ERa-negative cells from the normal breast immortalize in vitro. During an investigation aimed to tackle these difficulties, we made the following observations, which form the basis of this application. 1) ERa-negative mammary epithelial cells engineered to express ERa undergo senescence-like growth arrest similar to oncogene-activated senescence during premalignant stages. 2) Senescence-like growth arrest was accompanied with activation of p53 and the checkpoint kinase Chk2 and induction of the senescence-associated gene Cst1. 3) ERa- positive cells eventually re-entered cell cycle, which correlated with loss of Chk2 and p53 activation. But these cells fail to achieve ERa-E2-dependent proliferation suggesting additional signals for ERa-mediated proliferation. We hypothesize that there are at least two major steps in the initiation of ERa-positive breast cancers: 1) inactivation of the Chk2/p53 mediated senescence-like growth arrest pathway, 2) expression of genes such as FoxA1, which is required for ERa-mediated proliferation. Interestingly, brassica vegetable- derived che mo preventive agents such as phenethyl isothiocyanate (PEITC) activate Chk2, which may lead to senescence-like arrest of premalignant cells. Using the above in vitro assay system as well as a polyoma middle T antigen (PyMT) transgenic model system, we will 1) investigate the effect of PEITC on re-entry of senescent ERa-positive cells to cell cycle and acquisition of ERa:E2-dependent proliferation and 2) determine the effect of PEITC on Chk2 activation and senescence of ERa-positive cells in hyperplastic, adenoma, early and late adenocarcinoma stages in the PyMT transgenic model of breast cancer. Our long- term goal is to identify molecular targets for senescence-based chemoprevention strategies for ERa-positive breast cancers. [unreadable] [unreadable] [unreadable]